The desirability of sampling of flowing material for various purposes is well known, and such devices for such a purpose have been heretofore suggested and/or utilized. Utilization of a sampling device in particle measuring systems for optical sizing purposes is also now known, and such an optical particle measuring system is shown, for example, in U.S. Pat. No. 3,406,289.
It has been found that where a flow path includes a flow restriction, such as a nozzle or jet, the resulting increase in acceleration in flow through the inlet results in adiabatic cooling which adversely affects laminar flow. For sample flow through a nozzle in an optical particle measuring apparatus of the type having an open cavity laser, it has been found that the laser operates at a lower cavity Q (the ratio of the resonant energy density inside a laser cavity to all possible losses is referred to as the "Q" of the cavity) due to non-laminar flow characteristics since open cavity devices are very sensitive to such flow characteristics.